1. Field of the Invention
The present invention relates to a modulation circuit which translates each m-data-bits (=m-bit dataword) cut out of a binary data stream one after another into a n-code-bits (=n-bit codeword) respectively, then converts the translated RLL code data sequence into an NRZI (Non Return to Zero Inverted) code data sequence, and then outputs the NRZI code data sequence to an outer circuit. Also, the present invention relates to a demodulation circuit which demodulates an NRZI code data sequence into an RLL code data sequence, then decodes each n-code-bits cut out of the RLL code data sequence one after another into a m-data-bits respectively, and then outputs the decoded binary data sequence to an outer circuit.
2. Description of the Related Art
An input binary data stream is mapped to an RLL code data sequence, then modulated to an NRZI waveform, and then recorded onto a recording medium, and thereby the recording density can be increased.
In the RLL coding, m-bit datawords are cut out of the binary data stream one after another, and each m-bit dataword is mapped to an n-bit codeword. In this mapping or translating, a constraint is imposed to set the minimum value T.sub.min of the polarity inversion interval (=transition interval) of the NRZI code data sequence at a larger value and to set the maximum value T.sub.max thereof at a smaller value. Specifically, with respect to the RLL code data sequence, a constraint is imposed such that the number of bits 0's existing between the bit 1 and the adjacent bit 1 should be limited to be d or more and k or less. In other words, a constraint is imposed such that two logical 1's should be separated by a run of at least d consecutive zeros and any run of consecutive zeros has a length of at most k. The RLL code data sequence mapped to satisfy this constraint is called (d,k;m,n) RLL code.
In the NRZI modulation, an operation is performed such that the RLL czde data sequence is inverted only at the bit 1 and not inverted at the bit 0. In other words, the RLL code data sequence is converted into an NRZI waveform using a transition for a 1 and no transition for a 0. By such modulation or conversion, the minimum distance between transitions of the NRZI code data sequence becomes larger than that of the corresponding RLL code data sequence. Therefore, compared with a case where the RLL code signal before NRZI modulation is recorded onto a recording medium and read the same therefrom, the waveform distortion of the reading signal is reduced in a case where the NRZI-modulated code signal is recorded onto a recording medium and reading therefrom. As a result, the error rate of the reading signal can be lowered. If the same level of waveform distortion is acceptable in both the cases described above, the recording density can be heightened better in the case where the NRZI-modulated code signal is recorded onto a recording medium than in the case where the RLL code signal before NRZI modulation is recorded onto a recording medium.
In both the RLL coding and the NRZI modulation, the following conditions must be met:
(1) The minimum transition distance T.sub.min
When the recording density becomes greater, the minimum transition interval T.sub.min of the recording signal becomes smaller. Therefore, the reading pulses may be distorted due to the interference of the adjacent pulses, and as a result, reading errors may arise. To reduce the distortion of the reading pulses from a high density recording medium and thereby reduce the reading errors, the minimum transition period of time T.sub.min should preferably be large.
(2) The maximum transition distance T.sub.max
The reading pulse from a recording medium can not be obtained during the time period of no transition. Within that time period, the clock based on the reading pulse can not be generated, and therefore the clock may be inaccurate. On the other hand, when the transition interval of the recording signal is large, the DC component of the recording signal may become large. Therefore, the maximum transition period of time T.sub.max should preferably be small.
(3) DC component and low frequency component
Both the recording device and the reproducing device have AC coupling elements. The recording signal having the DC component or low frequency component is undesirably distorted through the AC coupling elements of the recording device, and this distorted signal is recorded onto a recording medium. As a result, the reading signal from the recording medium, which is recorded with the distorted signal, has inevitable distortion. Therefore, the DC component and low frequency component should preferably be small. The DC component or low frequency component of the recording signal is evaluated by using the DSV (digital sum value). The DSV is the sum of bit values from the specified start bit of the binary data sequence with the bit "1" taken as "+1" and the bit "0" as "-1." When the absolute value of DSV is small, the DC component or low frequency component of the binary data sequence is small. On the other hand, the DC component or low frequency component of each codeword is evaluated by using the CDS (codeword digital sum). The CDS is the sum of bit values from the start bit of the codeword to the end bit thereof with the bit "1" taken as "+1" and the bit "0" as "-1." When the absolute value of CDS is small, the DC component or low frequency component of the codeword is small.
(4) Window margin T.sub.w
The window margin T.sub.w indicates the allowable range of the phase shift of the reading signal due to interference thereof, noise, etc. The window margin T.sub.w is given by (m/n)T. Here, T refers to the length of I-bit data before RLL-coding. The window margin T.sub.w should preferably be large.
(5) Constraint length L.sub.c
To optimize the T.sub.min, the T.sub.max and the DSV, coding may be performed by referring to the codewords before or after thereof. The length of the codewords before thereof, datawords before or after to be referred to then is called "constraint length L.sub.c." The larger the L.sub.c is, the more extensive the error propagation is, and therefore, the L.sub.c should preferably be small.
In the Japanese Unexamined Patent Publication No. 52-128024, a technique to set the T.sub.min of the NRZI-modulated signal larger and to set the T.sub.max of the NRZI-modulated signal smaller is disclosed. According to the technique of the 52-128024, each 2-bit dataword serial/parallel converted from an input binary data stream is translated to a corresponding 3-bit codeword to generate a (1,7;2,3) RLL code. Then, the translated RLL code sequence is modulated to an NRZI code sequence. In such cases that the constraint d=1 is impossible to be satisfied in the above translation, a (1,7;4,6) RLL code is generated.
In the Japanese Examined Patent Publication No. 1-27510, a technique to reduce the DC component of the NRZI-modulated signal and to set the T.sub.min of the NRZI-modulated signal not smaller is disclosed. According to the technique disclosed in the No. 1-27510, the redundant data of a predetermined number of bits is inserted into between the adjacent n-bit codewords cut out of an Rll code data sequence one after another. Then, this inserted code sequence is modulated to an NRZI code sequence. Here, each redundant data is determined based on the necessity of the transition between the codewords and the state of the tail bit of the immediately preceding codeword. That is, the determination is made in such a way that the DC component of the NRZI-modulated signal can be reduced and the transition distance of the NRZI-modulated signal can not be set less than T.sub.min.
In Japanese Examined Patent Publication No. 5-34747, a translating or a coding method to set the T.sub.min at 1.5T, set the T.sub.max at 4.5T and set the L.sub.c at 5T is disclosed. According to the technique disclosed in the No. 5-34747, the regulation of the coding is determined based on the arrangement of the input binary data sequence.
In the Japanese Examined Patent Publication No. 4-77991, a technique to reduce the DC component of the NRZI-modulated signal and to set the T.sub.min of the NRZI-modulated signal larger is disclosed. According to the technique disclosed in the No. 4-77991, each 8-bit dataword serial/parallel converted from an input binary data stream is translated to a corresponding 14-bit codeword to generate a (1,8;8,14) RLL code. That is, the conversion is performed in such a way that the number of bits 0's existing between the bit 1 and the adjacent bit 1 of the coded data sequence can be 1 or more and 8 or less. The table for use in the coding of the 8-bit dataword to the 14-bit codeword is made ready for 2 different types, and depending on the DSV of the tail bit of the codeword coded immediately before, the codeword in either of the tables is selected. That is, the selection of the table is made in such a way that the DC component of the NRZI modulated recording signal can be reduced.
In the Japanese Unexamined Patent Publication No. 6-311042, a technique for heightening the recording density ratio DR (density ratio) by sufficiently reducing the DC component of the NRZI modulated recording signal and setting the T.sub.min of the NRZI modulated recording signal large is disclosed. According to this technique disclosed in the No. 6-311042, 8-bit datawords are cut out of the input binary data stream one after another and translated to respective 17-bit codewords. The translation or mapping is performed in such a way that the number of bits 0's existing between the bit 1 and the adjacent bit 1 can be 2 or more and 9 or less. The above-mentioned 17-bit codeword is generated by adding the redundant data of 2-bit to the 15-bit code corresponding to the original 8-bit dataword. According to the technique disclosed in the No. 6-311042, the table for use in the mapping of the 8-bit dataword to the 15-bit code is made ready for 2 different types, and the redundant data of 2-bit is made ready for 3 different types. The above-described 8-bit dataword is translated to the 17-bit codeword selected based on the DSV at the tail bit of the data coded immediately before from among six different types of codewords obtained by combining the 2 different types of tables and the 3 different types of redundant data. That is, by the 17-bit codeword selected in such a way that the DC component of the NRZI modulated code data can be reduced, the above-described 8-bit dataword is replaced.